Electrical insulating material and method of making

ABSTRACT

ELECTRICALLY INSULATING ARC-INTERRUPTING MATERIAL SUITABLE FOR MAKING CORES FOR CURRENT-LIMITING FUSES IS COMPOSED OF AN EXTRUDABLE MIXTURE OF HYDRATED ALUMINA, BENTONITE, AND AN INORGANIC BINDER SOLUTION SUCH AS ALUMINUM PHOSPHATE-PHOSPHORIC ACID. KAOLIN MAY BE ADDED TO THE MIXTURE TO INCREASE THE STRENGTH OF THE FINAL REACTION PRODUCT.

J. J. PlTHA 3,562,162

ELECTRICAL INSULATING MATERIAL AND METHOD OF MAKING Pau 3, l7! 1 Filed Nov. 14, 1966 a current limiting fuse having a fuse core 7 Claims ABSTRACT OF THE DISCLOSURE 'Electricall latin arc-interru tin material suitable for making cores for current-limiting Euses is composed of an extrudable mixture of hydrated alumina, Bentonite, and an inorganic binder solution such as aluminum phosphate-phosphoric acid. Kaolin may be added to the mixture to increase the strength of the final reaction product.

The present invention relates to electrical insulatingmaterial, and more particularly concerns electrically insulating, arc-interrupting ceramic material, and to a method of making the same.

The insulating material provided by the invention is particularly suited for use as a core in current-limiting fuses, in which good mechanical strength and electrical insulation properties, coupled with arc-interruption qualities and proper shape of the core, are essential for effective operation of the fuse.

It is an object of the invention to provide an improved electrical insulating material of the above type which has good mechanical strength.

-It is a particular object of the invention to provide a material of the above type which is entirely inorganic in composition.

It another object of the invention to provide an improved process for making insulating products of the above composition, especially products such as arc-interrupting fuse cores.

It is still another object of the invention to provide an improved process for making products of the above type wherein the products are made by extrusion forming methods.

Other objects and advantages will become apparent from the following description and the appended claims.

with the above objects in view, the present invention relates, in one of its aspects, to an electrically insulating arc-interrupting material suitable for extrusion forming comprising the reaction product of a mixture of hydrated alumina, bentonite, kaolin, and an inorganic binder solution, the latter preferably being a solution of aluminum phosphate in phosphoric acid.

In another of its aspects, the invention relates to the process of making the above-described material which comprises extruding the aforementioned mixture into desired shape, and heating the thus formed material within a particular temperature range, as disclosed more fully below, for hardening the same.

The invention will be betterunderstood from the following description taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a view in elevation, partly broken away, of which may be made in accordance with the invention;

FIG. 2 is a perspective view of the fuse core of the FIG. 1 device; and

FIG. 3 is a cross-sectional view of the FIG. 1 device taken along the line 33.

Referring now to the drawing, there is shown a current limiting fuse 1 in which the present invention may be embodied and which comprises an electrically insulat- United States Patent ing tube 2 closed at opposite ends by metal end caps 3 and 4 and enclosing therein a ceramic insulating core 5 (referred to in the art as a spider) on which is wound a fusible wire element 6 extending along the length of core 5. Arranged at opposite ends of core 5 in electrical contact with the ends of fuse element 6 are terminal clamp members 7, 8 which electrically connect fuse element 6 to the conductive end caps 3, [by means of outwardly extending. fingers, as shown, fuse core 5 is imbedded in sand 9 contained in tube 1.

As well understood by those versed in the art, fuse 1 operates as a protective device for electrical apparatus by limiting fault currents which may occur on electrical supply lines to which the apparatus is connected. The interruption of the fault current by operation of the fuse may result in arcing along the portion of the fuse wire 6 which has fused and vaporized due to the fault current.

For the purpose of interrupting the are thus produced, fuse core 5 is made of a type of electrical insulating material which evolves water vapor in the presence of the are, so as to cool and thereby extinguish the arc.

In accordance with the invention, core 5 is composed of an entirely inorganic ceramic material which has such vapor-generating properties and also has good mechanical strength to serve as a support for the fuse wire, is a good thermal conductor, and lends itself to ready forming by extrusion methods.

The composition from which such ceramic is made is as follows, the proportions of ingredients being given in percent by weight:

; Percent Hydrated alumina 60-85 Bentonite 5-15 Kaolin p 0-25 Inorganic binder in water solution 10-15 The hydrated alumina evolves water vapor when heated 'by an electrical arc in the fuse, so as to extinguish the 29.; nents to form a supp g rting network or mgt r igg gomposggl, statesman.ea hatealsninmflsilisnaw n amm is sgxt ains mmahvdrateaartig ss.

whsnoiadeesuhichamaythe mm t siligae sclutiontwatet. s amp, t g thq, ingmdiems of the n vrel ets 5221mm temrk arses t f silicate for the aluarmies...

K pa arly preferred composition which has been found to provide excellent results for current-limiting fuses of the described tym is as follows, in percent by weight:

' Percent Hydrated alumina 60 Bentonite (Wyoming) 6 Kaolin (Florida) 20 Mono-aluminum phosphate, 50% solution 14 acid whichisudesirahleio pggyide fg fugther g e action bEl ffifllhfi-p flfiphoricacidandaporfionofihehydratfid #l'lfli EJQ.enhance..theiormation.ot ..thendcscthed.sqrxstalline 9229. nsns orkzln a typ a h ndersolutionci. hajeyatx et zc.sPJuti n.co tsine.l.l2%..lz0.:and. 30-36% P In a typical process for making an insulating product from the above composition,-..=the alumina hydrate, bentonite and kaolin in required amountsare mixed-together,

the binder solution is added, and the mixing is continued until a homogeneous mixture, ,which;has,.the consistency of a stiff mud, is obtained. This mixtureis then passed through a suitable extrusionapparatus to form the mass into desired shape. As well understood by those skilled in the art, extrusion ,forming is achieved by forcing a material of suitable plasticity under pressure through a die of appropriate shape. The extruded material is first air-dried at room temperature and then oven-dried at about 100 C. for about one hour, after which the material is cut to the desired lengths. These parts are then subjected to final curing at about 250 C. to about 280 C. for one hour to harden them.

Heating the parts above the specified range entails the risk of irreversibly dehydrating the alumina hydrate, whereas heating substantially below that range may result in the parts having insufficient mechanical strength.

There is thus provided by the invention a fuse core which is entirely inorganic in composition. It thus avoids the disadvantages of known insulating products of this type containing organic materials which result in undesirable carbonization, and hence poorer insulating qualities, under the effects of arcing. Morover, the extruded structure provided by the invention is more porous than products of this type made by molding processes, and therefore it affords greater. surface area through which vapor evolution may take place for arc-extinguishing pur-' poses.

Although the invention has been described with respect to its particular application to current-limiting fuse cores,

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of making an electrically insulating arcextinguishing ceramic material which comprises providing a mixture in percent by weight of 60-85% hydrated. alumina, 5-15% bentonite and -15% of an inorganic binder in water solution, said inorganic binder solution selected from the group consisting of phosphoric acid solution of aluminum phosphate, and sodium silicate solu- 10 tion, extruding said mixture for forming the same, and

ture consists essentially of the following approximate composition in percent by weight:

Percent Hydrated alumina 60 Bentonite 6 Kaolin 20 Inorganic binder in water solution 14 6. A product as defined in claim 5, said inorganic binder solution being an aluminum phosphate-phosphoric acid solution.

7. A product as defined in claim 5, said inorganic binder solution being a sodium silicate solution.

References Cited UNITED STATES PATENTS 1,613,757 1/1927 Lindstrom 106-84 2,768,264 10/1956 Jones et al. 252-632 2,888,406 5/1959 Bondley et al. 2s2 51s 3,078,550 2/1963 Rakowski 264-61 OTHER REFERENCES Gregory, Uses and Applications of Chemicals and Related Materials, pp. -6 (1939), Reinhold Co.

JOHN D. WELSH, Primary Examiner US. Cl. X.R. 

